Methods and systems for making an electrode free from a polymer binder

ABSTRACT

The disclosure describes an exemplary binding layer formed on Aluminum (Al) substrate that binds the substrate with a coated material. Additionally, an extended form of the binding layer is described. By making a solution containing Al-transition metal elements-P—O, the solution can be used in slurry making (the slurry contains active materials) in certain embodiments. The slurry can be coated on Al substrate followed by heat treatment to form a novel electrode. Alternatively, in certain embodiments, the solution containing Al-transition metal elements-P—O can be mixed with active material powder, after heat treatment, to form new powder particles bound by the binder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims priority to, co-pendingU.S. patent application entitled “METHODS AND SYSTEMS FOR MAKING ANELECTRODE FREE FROM A POLYMER BINDER,” filed on Apr. 18, 2013, andassigned application Ser. No. 13/865,962, which is incorporated hereinby reference in its entirety.

TECHNICAL FIELD

The present disclosure is generally concerned with processing techniquesfor materials synthesis for lithium ion batteries.

BACKGROUND

A conventional process of making an electrode, which is a necessary partof secondary batteries, involves a step of applying a polymer binder soas to increase adhesivity between an electrode layer containing theactive material and a substrate, where the polymer binder binds thesubstrate with the active material.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of certain embodiments of the presentdisclosure. Moreover, in the drawings, like reference numerals designatecorresponding parts throughout the several views.

FIG. 1 is a diagram of an exemplary embodiment of a furnace and a heattreatment environment for the synthesis of electrode materials,including binder materials, in accordance with the present disclosure.

FIGS. 2(a)-2(b) are diagrams illustrating examination results for thecharge capacity of synthesized electrode materials in accordance withembodiments of the present disclosure.

FIG. 3 is a diagram illustrating electrochemical data resulting fromtesting of synthesized electrode materials in accordance withembodiments of the present disclosure.

DETAILED DESCRIPTION

Disclosed herein are certain embodiments of a technique in making anelectrode free from a polymer binder, such as for lithium ion batteryapplications. In accordance with one embodiment, good material/substrateand material's inter particle interfaces can be stabilized (bound) withthe utilization of the inorganic binder containing Al, one of thetransition metal elements, and phosphate ions. In certain embodiments,the transition metal element may be a combination of transition metalelements.

Since an exemplary inorganic binder may provide both bonding andelectronic conducting dual characteristics, it is therefore possible tocreate an electrode for Li-ion batteries without both polymer binder andcarbon black that may either decompose at high voltages or createunnecessary porosity (or chances in the loss of contact) of theelectrode.

FIG. 1 shows the design of a furnace and a heat treatment environmentfor the synthesis of the materials presently disclosed. FIG. 1 showsreaction vessel 1, which is open to air in furnace 2. The furnace isopen to the atmosphere at 3 a and 3 b so as to maintain substantiallyatmospheric pressure in the furnace. Flow of gases into or out of thefurnace is dependent on heating and cooling cycles of the furnace andchemical reactions taking place with materials in the furnace. Air isfree to enter the furnace, and air and/or products of a chemicalreaction of materials 4 in the reaction vessel 1 are free to exit thefurnace. Materials 4 in vessel 1 react chemically during heating stepsto form cathode materials in accordance with the present disclosure.Materials 4 in vessel 1, which face air found in the furnace, arecovered by a layer of a high temperature inert blanket 5, which isporous to air and escaping gases caused by the heating step. Heatingcoils of the furnace are indicated at 6.

The following are examples of exemplary techniques in accordance withembodiments of the present disclosure.

Example 1 Comparative Study of Electrodes Made with Conventional PolymerBinder and Electrodes Made Using Al—Mn—PO₄ Inorganic Binder

This example gives a comparison between electrodes made usingconventional coating process with the use of polymer binder andelectrodes made following Part I and Part II below.

Part I. LiMnPO₄—LiMn₂O₄ (1.8:0.1 in molar ratio) composite materialelectrodes made using conventional coating process:

For electrode preparation, active material (e.g., 5 g), Super P (e.g., 1g) and SBR (e.g., 0.3 g) was used in slurry making. After coating usingdoctor blade, the coated electrode was dried at 110° C. for 3 hoursfollowed by punching of the electrode. After vacuum drying again at 110°C. for overnight, the electrodes were transferred to the glove box fortest cell assembly, where the test cell is a three-electrode design withLi as the reference electrode. The electrode loading was determined tobe 6 mg and the active material content was 81.3%. The C-rate used wasapproximately C/10 (50 uA) and the room temperature was approximately23° C.

Referring to FIG. 2(a), a charge capacity of 160.5 mAh/g and a dischargecapacity of 51 mAh/g was obtained for the prepared electrodes, afteranalysis. Results also indicate that the corresponding Coulombefficiency was 31.7%. Since the test cell was charged to 4.9V, more orless decomposition of the electrolyte during charging could result inthe low Coulomb efficiency.

Part II. LiMnPO₄—LiMn₂O₄ (1.8:0.1 in molar ratio) composite materialelectrodes made using inorganic binder:

For substrate preparation, the following steps were followed.

-   -   1. Al plate (2.67*2.67 cm, with one side covered by a polymer        film) is used as the initial substrate.    -   2. Prepare 5M phosphorous acid/n-Butanol solution (23 g        phosphorous acid diluted by n-Butanol to 40 ml in volume) as the        etching solution. Warm the solution to 50° C. for later use.    -   3. Soak the Al plate in the 5M phosphorous acid/n-Butanol        solution for 2 minutes. Follow by rinsing the Al plate in 100 ml        n-Butanol for approximately 20 seconds.    -   4. Dry the Al plate at 60° C. in the drying oven (approximately        30 minutes).        Remarks: At this moment, Al—PO₄ thin film is formed as a film in        white color.    -   5. Spread MnO₂ powder through 250 mesh sieve on top of the Al        plate. Then, pass the as-made (MnO₂ coated) substrate through        the calendaring machine.    -   6. Heat treat the as-made substrate at 330° C. for 2 hours in        box furnace. After cooling, the as-made substrate is ready for        battery active material loading.        Remarks: At this moment, Al—Mn—PO₄ thin film is formed as a film        in brown color. The film is electrical conducting and can be        easily examined using the volt meter.

Next, for electrode preparation, the following steps are followed.

-   -   1. The above mentioned battery active material is loaded on top        of the as-made substrate by spreading the active material powder        through 250 mesh sieve.    -   2. Pass the as-made (active material loaded) electrode through        the calendaring machine again for compacting the electrode.    -   3. Send the as-made electrode to the box furnace and heat treat        at 330° C. for 4 hours under normal air atmosphere.    -   4. Punch the heat treated electrode and vacuum dry the samples        at 110° C. for overnight. The dried electrodes are then        transferred to the glove box for test cell assembly.

For the test cell, a three-electrode design with Li as the referenceelectrode was used. The electrode loading (14.6 mg) was calculated bytaking the weight difference between the before active materials loadingand the after heat treatment stages, then divided by the area of thesubstrate assuming the coating was even. The electrochemical test resultis shown in FIG. 2(b). Accordingly, the C-rate used (220 uA) was aroundC/10 and the room temperature was around 23° C.

Further, from FIG. 2(b), a charge capacity of 131 mAh/g and a dischargecapacity of 105 mAh/g was obtained. The corresponding Coulomb efficiencywas indicated to be 80%. Since the test cell was charged to 4.5V only,the charge capacity is less than the data using normal coating method.However, the increase in Coulomb efficiency may suggest the possibledecompositions of electrolyte and polymer binder have been minimized.

In this example, it is clear that the substrate to material interfacecan be improved with the use of Al—Mn—PO₄ film. If the Al—Mn—PO₄ film isnot electrically conducting, the electrochemical behavior should havebeen deteriorated. Nonetheless, since no polymer binder and carbon blackwere used in the electrode making process, the decomposition reactionsat high voltages have been minimized.

Example 2 Same as-Made Substrate Loaded with LFPO

In this example, the same as-made substrate (Al—Mn—PO₄ thin film coated)was used. The Lithium Iron Phosphorous Oxide (LFPO, U.S. Pat. Nos.7,494,744, 7,585,593, 7,629,084, 7,718,320) material manufactured byChangs Ascending Co. Ltd. was used as the active material. Again, thematerial was spread on the substrate by sieving the material through the250 mesh sieve. Then, calendaring was conducted on the as-made (activematerial loaded) electrode. After calendaring, the as-made electrode wassubjected to heat treatment at 330° C. in air for 4 hours.

The electrochemical data is shown in FIG. 3. From the figure, it can beseen that the test cell cycled very well although the initial cellvoltage was higher than normal during the first charge. It should bementioned that no polymer binder and carbon black was used in thisexperiment. This experiment again demonstrates the use of Al—Mn—PO₄ filmthat allows direct bonding between the materials to the substrate withgood electrical conductivity.

It is noteworthy that while spreading the active material on top of thesubstrate, the substrate can be wetted using either pure water (or othersolvents) or very dilute polymer solutions for preventing powder dropoff before calendaring. The polymer solutions, for example, can be CMCsolution (Carboxylmethyl cellulose, 0.01 wt %), or SBR solution (styrenebutadiene rubber solution, 0.005 wt %) but not limited to theseexamples. In any case, the solution containing solvent or organicmolecules will be vaporized or decomposed during subsequent heattreatment. Besides, calendaring again after heat treatment is exhibitingno harm to the exemplary materials of the present disclosure.

Example 3 Inter Particle Bonding Using Solutions Containing Al—Mn—P—O

From previous examples, it was realized that the source of aluminumsubstrate may not be the only source of Al. Furthermore, the bondingbetween the material and substrate can be extended to the cases such asbonding between inter particles of the active material. Bonding betweenactive material particles would result in the following benefits: i).Thicker active material films could result in higher volume energydensity of the final battery; and ii). More reliable electrical contactbetween particles would lead to more consistent battery performance andthus better cycle life. A variety of different sources for Aluminum wasthen explored as potential solutions as discussed below.

Solution 1: Al source from pure Al foil. In this example, the source ofAl was obtained by dissolving Al foil directly in phosphoric acid. Inone example, dissolving 22.5 g (0.83 mole) Al foil in 230 g phosphoricacid (2 mole) resulted in a ratio of Al:P=5:12. Then, a fraction of theAl—P—O solution was utilized in dissolving manganese formate. Thus, asolution containing Al:Mn:P=5:7:12 was obtained. This solution has beenutilized in making slurries containing the active materials. Aftercoating the slurry on the Al substrate, subsequent heat treatment at330° C. for 2 hours in oxygen or air can result in nice and firm coatedfilm ready for lithium ion battery assembly.

Solution 2: Al source from Al₂(SO₄)₃. In this example, Al₂(SO₄)₃ (e.g.,2.14 g) (0.00625 mole) was dissolved in 15 g water. Then, H₃PO₄, 1.44 g(0.0125 mole) was added to the solution. Finally, manganese formateMn(HCOO)₂ (e.g., 0.91 g) (0.00625 mole) was dissolved in the solutionthat resulted in the ratio of Al:Mn:P=1:1:2. Usually 50 g of activematerial (can be LiMn₂O₄, Li_(1/3)Ni_(1/3)Co_(1/3)MnO₂, LFP or LFPO) ismixed with the as-prepared solution and a slurry, or a paste, or wetpowders is formed. A convenient way to make a slurry is by adding properamount of water that can be coated on Al substrate, then followed by aheat treatment at 330° C. for 2 hours in oxygen or air. The electrodeloading can be as high as 50 mg/cm² with thickness more than 200 umwithout showing any peel off problems.

Solution 3: Formation of AlPO₄. In this example, an AlPO₄ compound wasfirst synthesized. After dissolving AlPO₄ in solvents such as water, thesolution was mixed with active material. In such occasion, thetransition metal source can be from trace elements in the activematerial. After heat treatment, still Al-Transition metal element-P—Ocan be present in the active material or between the activematerial/substrate interface. Exemplary synthesis routes for AlPO₄ aredescribed below:

-   -   1. Dissolve aluminum formate in phosphoric acid (1:1 in molar        ratio). Place the solution in a stainless steel crucible.        Bringing the sample to 475° C. for 1 hour can result in phase        pure AlPO₄.    -   2. Dissolve aluminum nitrate in phosphoric acid (1:1 in molar        ratio). Place the solution in a stainless steel crucible.        Bringing the sample to 500° C. for 15 minutes can result in        phase pure AlPO₄.    -   3. Disperse aluminum acetate in phosphoric acid (1:1 in molar        ratio). In this case, aluminum acetate cannot dissolve in        phosphoric acid fully with 1:1 molar ratio. Place the solution        in a stainless steel crucible. Bringing the sample to 500° C.        for 20 minutes can result in a mixture of alumina and AlPO₄.

From Example 3, several conclusions can be made:

-   -   1. Al—Mn—P—O solution can be created in any ratio.    -   2. The Al—Mn—P—O solution can be incorporated in the slurry        making stage and is ready for coating. Subsequent heat treatment        can result in nice and firm electrode.    -   3. The Al—Mn—P—O solution can be incorporated in the material        that results in the wet powder form. Subsequent heat treatment        can bring the material back in the powder form. And, the powder        can be processed using conventional coating process utilizing        polymer binder and carbon black.    -   4. In any of the examples shown in Example 3, the inorganic        binder consisting of Al-transition metal-P—O resulted in the        resultant electrode or materials.

Exemplary embodiments advantageously feature a binding layer formed onAl substrate that binds the substrate with the coated material (see,e.g., example 1 and example 2). Also, exemplary embodiments disclose anextended form of the binding layer. By making a solution containingAl-transition metal elements-P—O, the solution can be used in slurrymaking (the slurry contains active materials). The slurry can then becoated on Al substrate followed by heat treatment to form a nice andfirm electrode. Alternatively, the solution containing Al-transitionmetal elements-P—O can be mixed with active material powder, after heattreatment, to form new powder particles bound by the binder (see, e.g.,example 3), in certain embodiments.

In one embodiment, an exemplary electrode assembly contains an Alsubstrate, and slurry material coating layer formed on the Al substrate,wherein the slurry material contains active material and Al-transitionmetal elements-P—O binder material.

Any process descriptions should be understood as representing steps inan exemplary process, and alternate implementations are included withinthe scope of the disclosure in which steps may be executed out of orderfrom that shown or discussed, including substantially concurrently or inreverse order, depending on the functionality involved, as would beunderstood by those reasonably skilled in the art of the presentdisclosure.

It should be emphasized that the above-described embodiments are merelypossible examples of implementations, merely set forth for a clearunderstanding of the principles of the disclosure. Many variations andmodifications may be made to the above-described embodiment(s) withoutdeparting substantially from the spirit and principles of thedisclosure. All such modifications and variations are intended to beincluded herein within the scope of this disclosure and protected by thefollowing claims.

The invention claimed is:
 1. An electrode assembly comprising: an Alsubstrate of a battery electrode; a layer of inorganic binder film overa top surface of the Al substrate, wherein the inorganic binder film isformed of Al from the Al substrate, a source of P—O elements, and atleast one transition metal element; and a layer of lithium ion batteryactive material over the inorganic binder film layer of the Al substrateto form the battery electrode, wherein the inorganic binder film layerformed of the Al from the Al substrate, the source of P—O elements, andthe at least one transition metal element binds the lithium ion batteryactive material with the Al substrate of the battery electrode, whereinthe battery electrode has not been subject to a charge-discharge cyclewithin a lithium ion battery.
 2. The electrode assembly of claim 1,wherein bonding between the inorganic binder film layer and the lithiumion battery active material comprises bonding between inter particles ofthe lithium ion battery active material.
 3. The electrode assembly ofclaim 2, wherein the inorganic binder film layer is directly bonded tothe Al substrate.
 4. The electrode assembly of claim 1, wherein polymerbinder or carbon black is not used in the electrode assembly.
 5. Theelectrode assembly of claim 4, wherein the inorganic binder film layercontains Al—Mn—PO₄.
 6. The electrode assembly of claim 1, wherein theinorganic binder film layer contains Al—Mn—PO₄.
 7. The electrodeassembly of claim 1, wherein the inorganic binder film layer is directlybonded to the Al substrate.
 8. The electrode assembly of claim 1,wherein the transition metal element is a combination of transitionmetal elements.
 9. The electrode assembly of claim 1, wherein thelithium ion battery active material comprises LiMn₂O₄,Li_(1/3)Ni_(1/3)Co_(1/3)MnO₂, LFP or LFPO.
 10. The electrode assembly ofclaim 1, wherein the at least one transition metal element comprises Mn.